1. Field of the Invention
The present invention generally relates to data communications cables, and more particularly relates to electrical communications/data cabling, which is formed of single, multi-strand wire conductors, as opposed to twisted pair conductors, in a flat, as opposed to round, configuration.
2. Background Information
In Universal Serial Bus (USB) specification cables, the properties of the cable must be adapted to carry information in accordance with the outlined specifications for the particular cable, as well as to comply with the adapter plugs that accompany USB outlets. These specifications include, amongst others, desired or required transmission rates or bandwidths, voltage ratings, temperature ratings, insulation resistance, conductor resistance or impedance at specified temperatures. In some USB cables, shielding is provided and the conductors are generally configured of four wires, arranged in two insulated, twisted pairs of data transmission signal wires. Typically, these wires are twisted pairs of data transmission wires made of 26 or 28 American Wire Gauge (AWG). Usually another two wires are included, the first is a power wire and the second is a power ground wire, both typically 24 AWG. The power wire is usually designed to provide 500 milliamps at 5 Volts from a computer to a peripheral device, and can handle a maximum of 30 Volts rms. Higher quality USB cables, which include twisted, paired conductors and shielding, are generally capable of data transmission rates of 12 Mbps. A polypropylene thread sealer is filled around the four wires, including the two pairs of twisted conductors and the two power wires, thereby forming a round, cross-sectional shape, which is an easy shape for extrusion and wrapping with a shield. Higher quality cables are typically double shielded and include an aluminum foil/Mylar and a wrap shield, which is then covered with a copper alloy braid. Lesser quality cables typically contain only one shield. A 28 AWG drain wire may also be present. The drain wire is in conductive contact with the outer shield and is used to dissipate radio frequency interference (RFI) and electromagnetic interference (EMI). The outermost shield is then covered with polyvinyl chloride or other sheathing material. In general, these high transmission rate USB cables have a round, cross-sectional configuration.
There is another specification for USB cables wherein no shielding is provided. These cables do not incorporate twisted pairs of data transmission conductors, and as a result the communications rating is much, much lower, typically around 1.4 Mbps.
A recent standard promulgated by the USB Board is the USB 2.0 standard. This type of cable can handle high-speed device transmission data rates as high as 480 Mbps. A typical round USB cable, conforming to the USB 2.0 includes double shielded twisted pairs of conductors wire as shown in Prior Art FIG. 1. As can be seen in Prior Art FIG. 1, two twisted pairs of wires, which are used as the data conductors, along with power lines 1 and 2 encased within a round jacket formed of polypropylene thread. A circular configuration is used to facilitate easier placement of shielding and extrusion of the outer jacket. The prior art cable of FIG. 1 includes a jacket wrapped around each wire of the twisted pair conductors and around both power wires. Two additional layers of shielding are provided around the inner jacket of polypropylene thread, the first is usually an aluminized foil, and the second a braided shield. All of this is, in turn, encased within an outer jacket.
A second type of data communications cable commonly available is one that complies with a set of standards promulgated by the Institute of Electrical and Electronic Engineers (IEEE). These are the IEEE 1394 and IEEE P1394 standards for data cables in common use today. The properties of this cable must be adapted both to carry information in accordance with the outlined specifications, as well as to comply with the adapter plugs that accompany IEEE 1394 outlets. IEEE 1394 cable is generally configured to have two power wires and four data conductors, all of which are insulated. The four data conductors are each comprised of a pair of twisted wires, typically 26 or 28 AWG. The other two wires constitute a power wire and a power ground wire, typically 24 AWG. The power wires are utilized to power the component connected to the cable. In some embodiments, the component has its own source of power and does not need a cable having power wires. In such an embodiment, a cable containing only four pairs of twisted wires may be used. The conductors and the power wires are bundled together much the same as is shown in Prior Art FIG. 1 for a USB cable. A polypropylene thread filler is filled around the four conductors, and the power wires, if provided, thereby form a round, cross-sectional shape, which is easier for extrusion and wrapping with a shield. Higher quality cables are typically double shielded with an aluminum foil/Mylar wrap shield, which is then covered with a copper alloy braid. Lesser quality cables contain only one shield. A 28 AWG drain wire may also be present. The outermost shield is then covered with PVC or other sheathing. Cables manufactured to IEEE 1394 specifications usually have a round, cross-sectional shape. Like USB 2.0, IEEE 1394 data transmission cables can have quite high transmission, up to 400 Mbs.
There are some basic problems or drawbacks with each of these prior art cables. The first is that there is some manufacturing problems associated with control over impedance characteristics. The first is control of wall thickness for the insulating jackets encasing each of the wires in the twisted pair before they are twisted together. The second problem is controlling matched lengths of wires when they are being twisted, and the third is that twisted pairs, when flexed or bent, have a tendency to separate from each other. All of these issues affect impedance.
Next, are the costs and time required in the manufacturing process for the additional step of fabricating the twisted pairs of conductor wires prior to fabrication of the cable. The second is the generally round, cross-sectional configuration of each of these cables. While the data transmission characteristics and transmission rates for cables manufactured to these specifications can, and are routinely met, round, sectional shaped cables have certain inherent limitations regarding their use. The primary limitation of the round cable is the fact that it is not amendable to being wound around a spool in a tight, compact configuration. A better configuration would be a shielded flat electric cable such as that is disclosed in the patent to King (U.S. Pat. No. 4,404,424), which issued Sep. 13, 1983. However, the problem with the cable disclosed in the King patent is that it still has the manufacturing drawbacks of the twisted pair configuration for the conductor wires.
An ideal cable would be a flat, shielded data transmission cable that meets all of the required data transmission specifications and rates, but which does not require the twisted pairs of conductor wires. In practice, it has been found that a four fold increase in production rates for data transmission cables can be achieved by co-extruding two conductor wires in parallel spaced relationship to each other, as opposed to individually coating each wire and twisting the two wires of each twisted pair together. This results in substantial manufacturing cost savings.
An additional benefit of the present invention is that a flat cable can be compactly wound around a spool such as those disclosed in U.S. Pat. Nos. 5,655,726 and 5,797,558.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.